Add new LLVM_OPTIMIZED_TABLEGEN build setting which configures, builds and uses a...

Add new LLVM_OPTIMIZED_TABLEGEN build setting which configures, builds and uses a release tablegen build when LLVM is configured with assertions enabled.

Summary: This change leverages the cross-compiling functionality in the build system to build a release tablegen executable for use during the build.

Reviewers: resistor, rnk

Reviewed By: rnk

Subscribers: rnk, joker.eph, llvm-commits

Differential Revision: http://reviews.llvm.org/D7349

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@231842 91177308-0d34-0410-b5e6-96231b3b80d8